5o8 SCIENCE PROGRESS 



substituted by which a continuous trace can be obtained. This 

 may, if required, be at some distance from the furnace, 



A simpler form of the Fery instrument is now made, in 

 which for the thermocouple and registering galvanometer is 

 substituted a very small bimetallic spiral actuating a long 

 aluminium pointer, moving over a scale graduated directly in 

 temperature degrees. Both of these instruments are arranged 

 to give readings independently of their distance from the emitting 

 source, provided the latter is large enough to give in the instru- 

 ment an image of sufficient size to cover the receiving disc. 

 The readings are independent of the personal element introduced 

 by the observer, no question of individual judgment as to 

 equality of brightness or colour-matching being involved. On 

 the other hand the disadvantages of the total radiation instru- 

 ment are that it requires a larger area of uniform temperature 

 upon which to focus than the other types of optical pyrometer, 

 and that no glass, mica or other absorbing screen may be 

 interposed in the path of the rays, unless special arrangement 

 be made to determine for each temperature the considerable 

 absorption thus caused. 



Other total radiation pyrometers are those of Thwing and 

 of Foster. Though the details in these are quite different, they 

 are both constructed on the same principle as the electrical type 

 of Fery instrument just described, but are somewhat simpler 

 and cheaper, no sighting or focussing device being considered 

 necessary. 



Another and older type of optical pyrometer is the Le 

 Chatelier in its original form, and as modified by Fery, the 

 latter being known as the Fery Absorption Pyrometer. 

 In these instruments and also in that of Wanner only light 

 of one wave-length is used for pyrometric measurements. All 

 three are photometers, in which comparisons are made of the 

 intensity of the red light coming from the hot body, whose 

 temperature is to be measured, with that from a standard 

 lamp of some form or other. A system of lenses forming a 

 telescope of low magnifying-power is used and the measurement 

 consists in adjusting to equality two patches of light, which 

 appear simultaneously in the field. In the Le Chatelier instru- 

 ment, this adjustment is made by altering the size of an iris 

 diaphragm ; in the Fery by sliding past one another two 

 graduated absorbing wedges. In both these instruments a real 



